1. FIELD OF THE INVENTION
This invention relates to a gas component detector suitable for use in an exhaust gas purifying system of the feedback type employing a three-way catalyst.
2. DESCRIPTION OF THE PRIOR ART
Hitherto, as a gas component detector for internal combustion engines, there have been several detectors which employ a sintered body of a metal oxide in which an electrical resistance value changes depending upon a partial pressure of oxygen in the environment. Recently, as a countermeasure against the polluting components in the exhaust gases of the internal combustion engine of automobiles, etc., purifying systems using a three-way catalyst for removing simultaneously the harmful components, CO, HC, and NO.sub.x, have been considered. Among these purifying systems one is a suction control system which controls the amount of suction air or fuel at the suction side, and another is an exhaust control system in which the air-fuel ratio (hereinafter referred to as A/F ratio) at the suction side is set richer than the ideal A/F ratio and secondary air is supplied to the exhaust gases, and the amount of the secondary air is controlled. In both cases, in order to enable the three-way catalyst to function effectively at a maximum degree, it is necessary to control the A/F ratio either at the suction side or at the exhaust side so as to keep the A/F ratio in a very narrow effective range, W, of the three-way catalyst as shown in FIG. 1, and for this purpose a sensor is necessary to sense the gas component of the exhaust gases and to detect the A/F ratio thereof. And by the use of the output signal of the sensor, the amount of air or fuel at the suction side is controlled, or the amount of secondary air supplied to the exhaust gases is controlled thereby to maintain the A/F ratio within the effective range of the three-way catalyst (W in FIG. 1), and thereby to eliminate the harmful components in the exhaust gases.
However, the gas component detector using a metal oxide among the above-mentioned gas component detectors, has the disadvantage that the metal oxide itself will be reduced (deoxidated) over a long duration in a reducing atmosphere at high temperatures, and also the electrical resistance in the oxidating atmosphere side will drop to a large extent resulting in a change of the inherent properties thereof. Furthermore, there is another disadvantage that when this gas component detector is used in the exhaust control system, a position at which the characteristics change rapidly will be shifted from the stoichiometric A/F ratio to the lean side largely as shown in FIG. 2 and hence it will be impossible to make the three-way catalyst function effectively.
The problem that the forementioned gas component detector is reduced in a high temperature and reducing atmosphere is supposed to be due to the existence of reducing gases (H.sub.2, CO, HC) and carbon. In other words, although the reducing gas itself has a reducing property to some extent, if the carbon coexists therewith, the reducing property is doubled, and results in a phenomenon in which a detector element of metal oxide is reduced at high temperatures and an electrical resistance value at the oxidating atmosphere side drops. Furthermore, a main cause for the forementioned disadvantages in the exhaust control system is considered to be due to the effect of unburned components, specially, H.sub.2 and CO in the exhaust gases. FIGS. 3a, 3b and 3c show the characteristics of the detector in the model gases of the H.sub.2 -air system, CO-air system and i-C.sub.4 H.sub.10 -air system respectively. As will be seen from these figures, both H.sub.2 and CO change their characteristics suddenly in the oxidating atmosphere side, and especially the effect of H.sub.2 is significant. Since the A/F ratio in the suction side is set to be rich in the exhaust control system, as will be seen from FIG. 4, H.sub.2 exists abundantly. In this case, if the catalytic ability of the gas component detector is superior, the gas component as shown in the FIG. 4 will act with the remaining oxygen satisfactorily to arrive at an equilibrium and the characteristics will be changed suddenly at a point of the stoichiometric A/F ratio. However, since the catalytic ability of the gas component detector is so small, the equilibrium is not achieved and causes a shift of the position at which the characteristics change suddenly. On the other hand, in the suction control system, since the control is effected in a region where H.sub.2 scarcely exists, the gases easily reach the equilibrium and the characteristics change suddenly at the point of the stoichiometric A/F ratio even if the catalytic ability is small.